The present invention relates to the removal of sulfur containing gases from gaseous fuel mixtures and the regeneration of sorbents used in this removal. The invention particularly relates to the regeneration of sorbents used for the high-temperature removal of hydrogen sulfide from fuel gases that are to be employed in fuel cell use for the generation of electricity.
Carbonaceous fuel and feedstock gases including both natural gas and producer gas often include sulfur-containing components such as hydrogen sulfide that may poison catalysts involved in conversion or utilization processes. This is particularly true in the case of feed gas for high-temperature, electrical fuel cells. Electrodes including nickel and other substances having important catalytic activity may be affected adversely by the presence of hydrogen sulfide in the fuel.
Some commercial methods for removing hydrogen sulfide from carbonaceous fuel gases involve scrubbing the gas stream with liquid solutions at a relatively low temperatures of about 100.degree.-150.degree. C. Such low temperatures are disadvantageous in processes for cleaning fuel for high-temperature applications such as molten carbonate and solid oxide fuel cells. Such fuel cells may operate at temperatures of 500.degree.-1000.degree. C. or above.
Various sorbents such as copper oxide, zinc oxide, iron oxide and other transition metal oxides have been employed at high temperatures for removing hydrogen sulfide from fuel gases. This spent sorbent contains sulfides of the metal and must be regenerated to the oxide prior to continued use or discarded. Regeneration by roasting in air or oxygen can be slow, costly and possibly sinter the sorbent material to reduce its porosity and effectiveness. Steam also has been used as a desorbing gas but with minimal effectiveness unless followed by oxidation with air or other oxygen-containing gas.
In the regeneration of such materials at high-temperatures by roasting in air or oxygen exothermic reactions occur that increase the risk of sintering. Severe loss of specific surface area can occur rendering such processes disadvantageous in the regeneration of spent solid sorbent material.
These previous methods are illustrated in the U.S. Pat. Nos. 3,974,256 and 4,002,720 to Wheelock et al. In these patents a spent lanthanide oxide sorbent is treated in a two step process involving initial desorption with steam followed by regeneration with an oxidizing gas. In these processes the steam desorption alone was found to be ineffective. Additional treatment with oxygen or air at elevated temperatures was required to effectively regenerate the sorbent.